1) Field of the Invention
The present invention relates to differential quadrature phase-shift modulator suitable for use in a transmitter of an optical transmission system, as well as to a method for setting a driving voltage of the differential quadrature phase-shift modulator.
2) Background of the Invention
In recent years, increasing demand has arisen for introduction of a 40-Gbit/s optical transmission system of the next generation. In addition, a transmission distance and frequency utilization efficiency equivalent to those achieved by a 10 Gbit/s system have been sought. In an effort to meet such demands, brisk research and development has been carried out on a DPSK (Differential Phase Shift Keying) modulation scheme, which is superior to an NRZ (Non-Return to Zero) modulation scheme having hitherto been applied to a system of 10 Gb/s or less in terms of optical-signal-to-noise ratio (OSNR) resistance and nonlinear resistance. In addition to research and development of the above-described modulation scheme, research and development of a phase modulation scheme called DQPSK (Differential Quadrature Phase-Shift Keying) modulation (differential 4-phase-shift modulation) having a feature of a narrow spectrum (high-frequency utilization efficiency) has also become brisk.
Particularly, the DQPSK modulation scheme is for simultaneously transmitting two phase-shift digital signals through use of signal light of a single frequency. According to this scheme, a pulse-repetition frequency (e.g., 20 GHz), which is one-half the speed (e.g., 40 Gbit/s) of data to be transmitted, is sufficient, and hence a spectrum width of a signal is reduced to one-half that achieved by the conventional NRZ modulation scheme. Thus, the DQPSK modulation scheme is superior in terms of frequency utilization efficiency, wavelength dispersion resistance, and a device transmission property. For these reasons, in the field of optical transmission systems, application of this modulation scheme to a high-speed transmission system whose data speed exceeds 40 Gbit/s has been actively investigated.
FIG. 21 is a view showing a BPSK modulator 110. The BPSK modulator 110 shown in FIG. 21 is provided in, e.g., an optical transmitter which transmits an optical signal in an optical transmission system and is for modulating a data signal into an optical signal by means of the DPSK modulation scheme. The BPSK modulator 110 has a transmission data processing section 111, an amplifier 112, a CW (continuous wave) light source 113, and a phase modulator 114.
The transmission data processing section 111 is for performing transmission data processing, such as encoding, or the like, of data to be transmitted. The data signal output from the transmission data processing section 111 is amplified by the amplifier 112, and the thus-amplified data signal is input to the phase modulator 114. The phase modulator 114 subjects the continuous light output from the CW light source 113 to phase modulation by use of the encoded data input from the transmission data processing section 111 by way of the amplifier 112.
The phase modulator 114 comprises a Mach-Zehnder waveguide 114a, and modulation electrodes 114b-1, 114b-2 formed on respective waveguides which have been bifurcated by the Mach-Zehnder waveguide 114a. The phase modulator 114 further comprises a π phase-shift section 114c formed in a stage subsequent to the modulation electrode 114b-2 on one of the waveguides bifurcated by the Mach-Zehnder waveguide 114a. 
Here, the continuous light (see FIG. 22A) that has exited the CW light source 113 and propagates through an upper waveguide, in the drawing, of the two waveguides bifurcated by the Mach-Zehnder waveguide 114a is modulated by a drive signal (a data signal output from the transmission data processing section 111) to be applied to the modulation electrode 114b-1. A phase component “0” is as signed to data “0,” and a phase component “π” is assigned to data “1”, whereby the light becomes an optical signal (see FIG. 22B and EU therein).
The continuous light (see FIG. 22A) propagating through a lower waveguide, in the drawing, of the waveguides bifurcated by the Mach-Zehnder waveguide 114a is modulated by a drive signal (an inverted signal consisting of the data signal applied as a drive signal to the modulation electrode 114b-1) applied to the modulation electrode 114b-2. The thus-modulated signal is then subjected to phase-shifting by a phase π in the π phase-shift section 114c. The phase component “0” is assigned to data “0”, and the phase component “π” is assigned to data “1”, whereby the light becomes an optical signal (see FIG. 22C and EL therein).
Thereby, the optical signals EU, EL propagating through the bifurcated waveguides that form the Mach-Zehnder waveguide 114a are merged, so that an optical signal whose light intensity is constant and whose information is superimposed on binary optical phases (0 and π); namely, an optical signal EOUT having been subjected to BPSK modulation, can be output as shown in FIG. 22D.
Next, there is shown an overview of a common configuration for transmitting data through the modulation and demodulation complying with the DQPSK scheme. Details of the configuration are described in, e.g., Published Japanese Translation of a PCT Application, No. 2004-516743, as well.
FIG. 23 is a view showing a common DQPSK modulator 130. The DQPSK modulator 130 shown in FIG. 23 is also provided in an optical transmitter, and modulates the data signal into an optical signal by means of the DQPSK modulation scheme. The DQPSK modulator has a transmission data processing section 131; amplifiers 132-1, 132-2; a CW (Continuous Wave) light source 133; a π/2 shifter 134; two Mach-Zehnder phase modulators 135-1, 135-2; and an MZM interferometer 136 for causing interference between the phase-modulated signals output from the phase modulators 135-1, 135-2, which differ from each other by a phase of π/2.
Specifically, the CW light source 133 is connected to an input side of the MZM interferometer 136, and phase modulators 135-1, 135-2 are formed in the respective bifurcated waveguides. In the following descriptions, the Mach-Zehnder waveguide forming the MZM interferometer 136 is sometimes described as a master MZ (Mach-Zehnder) waveguide. Like the phase modulators 135-1 and 135-2, Mach-Zehnder waveguides forming the phase modulators made in the bifurcated waveguide sections constituting the master MZ waveguide are sometimes described as slave MZ waveguides.
Here, the transmission data processing section 131 has the function of a framer or an FEC encoder, as well as the function of a DQPSK precoder which effects encoding operation reflecting encoding information about a difference between the code of current data and the code of data preceding the current data by one bit. The transmission data signal output from the transmission data processing section 131 is output as signals which are separated into encoded data of 20 Gbit/s of two channels (data #1, data #2), in connection with the encoded data of 40 Gbit/s. Alternatively, the amplifiers 132-1, 132-2 amplify data #1, data #2 of the encoded data and output the amplified data as drive signals to the phase modulators 135-1, 135-2.
Although the CW light source 133 outputs continuous light, the continuous light output from the CW light source 133 is bifurcated by the bifurcated waveguides forming the MZM interferometer 136. One light beam of the bifurcated light beams is input to the phase modulator 135-1, and the other light beam is input to the phase modulator 135-2. Each of the phase modulators 135-1, 135-2 has a configuration basically analogous to that of the phase modulator 114 shown in FIG. 21.
Here, as in the previously-described case shown in FIG. 22, the phase modulator 135-1 modulates the continuous light output from the CW light source 133 (see FIG. 25A) by use of the encoded data set (data #1) of one channel output from the transmission data processing section 131, thereby outputting an optical signal whose information is superimposed on a binary optical phase (0 rad or π rad) (see FIG. 25B).
Moreover, the phase modulator 135-2 modulates the continuous light from the CW light source 133 (see FIG. 25A) by use of the encoded data set of the other channel (the data #2) output from the transmission data processing section 131, and the thus-modulated optical signal is subjected to phase-shifting by φ=π/2 in the π/2 shifter 134. As a result, an optical signal whose information is superimposed on the binary optical phase (π/2 rad or 3π/2 rad) is output (see FIG. 25C).
The modulated light beams output from the above-described phase modulators 135a, 135b are merged by a merging waveguide forming the MZM interferometer 136, and the thus-merged light is output. Specifically, as a result of the modulated light beams output from the phase modulators 135-1, 135-2 being merged together, an optical signal which has constant light intensity and whose information is superimposed on optical phases of four values (π/4, 3π/4, 5π/4, and 7π/4); namely, an optical signal having been subjected to DQPSK modulation, can be output.
As mentioned above, during DQPSK modulation, digital signals of two channels, whose data “0 and “1” have been modulated into phase 0 and phase π, are caused to interfere with each other while being shifted from each other by π/2, to thus effect optical transmission by use of symbols of four values π/4(0, 0), 3π/4(1, 0), 5π/4(1, 1), and 7π/4(0, 1). In this case, when a change has arisen in the values of the data of two channels (a change from “0” to “1” or from “1” to “0”), the phases of the modulated light are switched. Therefore, dips arise in the intensity of light as designated by A to D shown in FIG. 24.
For instance, when both values of the data sets of two channels change (see B which changes from 5π/4 to π/4 in FIG. 24 and D which changes from 7π/4 to 3π/4 in FIG. 24), there arise comparatively large dips in intensity, which lowers to the neighborhood of a low level. In contrast, when only one value of the two data sets of two channels changes (see A which changes from 3π/4 to 5π/4 in FIG. 24 and C which changes from π/4 to 7π/4 in FIG. 24), there arise comparatively small dips in intensity, which lowers to the neighborhood of an intermediate value between a high level and the low level).
A known technique relevant to the present invention is described in, e.g., Patent Document 1.
(Patent Document 1) JP-A-2001-324732
However, as shown in FIG. 26, in the above-described DQPSK modulator 130, variations in manufacture of the MZM interferometer 136 often induce an imbalance between the intensity of the optical signal phase-modulated by the phase modulator 135-1 (an optical signal component P1 output from the phase modulator 135-1) and the intensity of the optical signal phase-modulated by the phase modulator 135-2 (an optical signal component P2 output from the phase modulator 135-2), both intensities being acquired when the optical signals are merged by the MZM interferometer, (P1≠P2). In this case, despite a phase difference between the symbols of four values assuming a value of π/2 in an ideal state, the phase difference deviates from π/2, to thus give rise to a deviation α. Therefore, there is a problem of the deviation entailing deterioration of signal quality.
Such an imbalance arises between P1 and P2 in a case where the extinction ratio of the modulator 130 is not good. Specifically, the imbalance is caused by a merging ratio of the merging waveguide and a bifurcating ratio of the bifurcating waveguide, both waveguides constituting the MZM interferometer 136, an imbalance between losses of the waveguides, and an imbalance between insertion losses of the Mach-Zehnder waveguide forming the two phase modulators 135-1, 135-2.
FIG. 27 is a graph showing the degree of signal deterioration (Q penalty) in relation to an extinction ratio (ExRp) of the Mach-Zehnder modulator formed from a substrate consisting of lithium niobate. A loss imbalance in an upper domain with reference to the horizontal axis represents reference values for the case where deterioration of the extinction ratio is attributable solely to an imbalance in the losses of the waveguides. As can be seen from FIG. 27, reducing the extent of deterioration of the signal requires a very high extinction ratio.
The technique described in Patent Document 1 set forth is provided for the configuration of an optical switch having an thermo-optic phase shifter which controls the thermo-optic phase shifter so as to provide an extinction ratio corresponding to the temperature of the substrate. However, the technique does not disclose any configuration for making signal quality of the DQPSK modulator excellent in response to an individual difference between extinction ratios due to variations in manufacture of devices.